Method of treating steel and novel product



June 6, 1967 A. LESNEY METHOD OF TREATING STEEL AND NOVEL PRODUCT Filed Sept. 15. 1964 IN V6 1V TOR ANDREW LESNEY Aflamey United States Patent 3,323,953 METHOD OF TREATING STEEL AND NOVEL PRODUCT Andrew Lesney, Frazer Township, Allegheny County, Pa.,

assiguor to United States Steel Corporation, a corporation of Delaware Filed Sept. 15, 1964, Ser. No. 396,765 8 Claims. (Cl. 148-39) This invention relates to a method of treating steel and to a novel steel product. The invention can be used to produce a steel strip having a ductile surface and a high-strength core. The term strip as used herein includes strip, sheet, plate, etc., without regard to thickness or length.

Steel strip products are currently being furnished in either the ductile, annealed condition with a recrystallized grain structure (which may also be temper rolled) or as high-strength, unannealed material with a cold worked metallurgical structure. Present day applications typically involve a compromise of ductility for high-strength or vice. versa. A sizable proportion of the steel strip produced is used as tin plate in the manufacture of can and container bodies. Where extensive forming or fabrication is necessary, as in container making, steel strip must be ductileenough to Withstand the forming operations without cracking. However, the containers themselves must meet rigid strength requirements.

The .present invention provides a method of making a steel strip with a novel metallurgical structure such that the strip possesses improved ductility for forming operations while maintaining high-strength properties. The novel structure is produced by starting with a rimmed steel having maximum carbon and manganese contents of 0.05% and 0.15%, respectively, and cold reducing the steel to desired strip gauge. The cold reduced strip is annealed under conditions which cause substantial surface recrystallization without substantially recrystallizing the core of the strip.

It is possible to improve the ductility of high-strength, i.e. partially or fully hardened strip, by annealing. However, when conventional partial or full hard strip is annealed, recrystallization occurs randomly throughout the strip with the result that the strength is materially reduced. Annealed black plate is an example of soft steel strip with good forming characteristics but relatively low strength. Black plate is a cOld reduced product 0.0141" or less that is not really black and is used in making tin plate and for other sheet and plate applications. Similarly, high-strength steel can be furnished even in very light gauges; however, steel strip of such material is generally too hard to withstand extensive forming operations without cracking. Unannealed cold reduced black plate and so-called thin tin plate are examples of such material. Thin tin plate is made by subjecting steel strip to a double cold reduction which not only decreases the thickness significantly (to about 0.0044") but also increases its hardness and tensile strength considerably. The present invention involves a discovery of a method of making steel strip which has a recrystallized surface that gives the strip greater ductility than unannealed high-strength strip in a fibrous, i.e. cold worked, core that results in a greater tensile strength than conventional annealed strip. It has been discovered that by using a rimmed steel with the maximum carbon content of 0.05 and a maximum manganese content of 0.15% and a controlled heat treatment, it is possible to anneal cold reduced strip of such composition so as to preferentially recrystallize the surface while leaving the core substantially unrecrystallized. The cold reduced, rimmed steel surface is recrystallized by annealing at a temperature in the range of 800 to 1150 F. for a time sufficient 3,323,953 Patented June 6, 1967 to substantially recrystallize the cold reduced rim of the strip but insuflicient to substantially recrystallize the cold reduced core of the strip.

The term rimmed steel as used herein refers to steel which has been solidified in a mold in such a Way that a rim of metal purer than the core exists at the surface of the ingot. Such steel is usually tapped without deoxidizing additions to the furnace in order to have sufficient oxygen present to react with carbon in the steel when the steel is poured in a mold. The carbon-oxygen reaction evolves gas and, as the metal in the ingot mold begins to solidify, the evolution of gas intensifies along the mold Wall surface. The result of this gas evolution is the formation of an outer ingot skin or rim of relatively pure, clean metal. Since freezing or metal solidification takes place from a layer of relative pure liquid, the entire solidified rim zone is composed of iron which contains a relatively small proportion of residual and alloy elements. When, after solidification, the rimmed steel ingot is rolled, to strip for example, the rim persists at the surface. The relatively ure metal in the rim provides excellent rolled surface quality Which is characteristic of production from rimmed steel ingots.

Steel strip is manufactured for tinplate stock by hot rolling, e.g. at above 1500 F., slabs which range from 4 to 7 inches in original thickness to a single continuous length of hot band gauge, i.e. about 0.080" which after descaling is coiled for handling. Of course, specific gauges and practices depend upon the intended use of the end product. To produce the usual commercial strip product, i.e. black plate, for thin coating, the coils are continuously pickled and then cold reduced to final gauge. The usual procedure is to cold reduce the strip to and then clean the strip to prepare it for annealing and tin coating. The cold reduced black plate is quite hard and for most applications must be softened by annealing. Both batch, i.e. box annealing and continuous annealing are used. In box annealing steel strip for tin plate the strip is usually heated in coil form for from 4 to 12 hours at 1l50 to 1250 F. Continuous annealing may be performed at rapid rates, e.g. 1000-2000 ft./min. by heat treating uncoiled strip continuously. After annealing the steel strip is usually temper rolled to improve its finish, flatten the strip and improve mechanical properties. The temper rolled strip is pickled and then may be tin coated by hot dipping or by electroplating. In hot dipping, the strip is passed through a bath of molten tin where it picks up a coating of the metal which solidifies upon cooling. Electrolytic tinning can be performed in either acid or alkaline electrolytes in Which the strip is made cathodic and metallic tin is deposited on the strip surface.

Double reduced tinplate stock is made by subjecting the cold reduced steel to a second cold reduction after an intermediate annealing which is usually conducted at about 1250 F. to fully recrystallize the steel. The sec ond cold reduction decreases the strip about 40% and greatly increases the strength as Well as the hardness.

The improvement of my invention is applied by manufacturing steel strip as above but using, however, a rimmed steel containing not more than 0.05% carbon and 0.15 manganese and performing a controlled heat treatment. The rimmed steel ingot is rolled to slabs and then hot rolled above 1400 F., preferably 1400 to 1600 -F., to the usual hot band gauge and coiled. The coil of rimmed steel is then cold reduced to the desired thickness after which it is annealed at a temperature in the range of 800 to 1150 F. for a time sufficient to recrystallize the cold reduced rimmed surface of the strip, but insuificient to recrystallize the strips core which maintains its cold reduced fibrous metallurgical structure. It is important to 3 observe the annealing temperature range, particularly the upper limit, ie .1150 F. If higher temperatures are used, the strip does not undergo preferential recrystalli zation but experiences random recrystallization of its mass. If temperatures less than about 800 F. are used;

annealing for surface recrystallization requires an unduly long period of time.

It is preferred to have 100% of the strip rim recrystallized by the annealing treatment with as little as possible of the core recrystallized. However, strip satisfactory for many purposes may be made with only 50% of the surface recrystallized and it is also possible to tolerate a small amount of recrystallization in the core. The extent of recrystallization that can be tolerated in the core, generally less than 30%, depends upon the strength of the final product desired since as the recrystallization increases, the strength decreases. In any event, it is necessary in practicing the invention that the surface or rim of the strip be recrystallized to a greater extent than the core.

The preferential surface recrystallization of rimmed steel strip is a function of time and temperature. Lower temperatures can be employed in box annealing where the strip is maintained at annealing temperature for relatively long time periods whereas higher temperatures are required for continuous annealing because of the short holding time permissible in the continuous operation. Thus, for example, rimmed steel strip can be preferentially surface recrystallized by box annealing at 850 to 950 F. in 4 to 30 hours or by continuous annealing at 1050 to 1150 F. in as little as to 40 seconds.

Preferential surface recrystallization of steel strip can be obtained only with a rimmed steel whose carbon and manganese content are limited to 0.05 and 0.15% respectively, and when annealed as described above. Steels whose compositions exceed these limits recrystallize randomly and not preferentially at the surface. Table I summarizes the results of a series of tests conducted on two cold reduced rimmed steels. Sample 1 contained 0.03% carbon and 0.08% manganese, whereas Sample 2 had 0.04% carbon and 0.18% manganese. The steel samples were annealed at different temperatures for various time periods and the percent of recrystallization in both the rim and the core was determined for each of the time and temperature conditions. It will be observed that recrystallization did not occur preferentially at the surface in steel Sample 2 containing 0.18% manganese. A similar effect is observed when the carbon content exceeds The steel samples used in conducting the tests reported in Table I were small specimens (1" x 2") cut from a sheet of 90% cold reduced (full hard) black plate 0.0066 thick. The specimens were heat treated in molten salt at the temperature and for the time periods indicated in Table I and then cooled in air. Rockwell hardness tests and metallographical examinations were made to determine the hardness and to estimate the percent of the recrystallization. The composition of steel Samples 1 and 2 are as follows:

The micrographs shown in FIGURES 1 and 2 are 250 magnification after a nital etch of the cross sections through preferentially recrystallized rimmed steel plates showing the recrystallized grains at the surface and the cold-Worked fibrous structure at the center. The steel strip after preferential surface recrystallization contains a fibrous cold worked core about 0.00 thick sand- Wiched between two surface layers about 0.001" thick of' completely recrystallized ferrite. The strip whose sections are shown in FIGURES 1 and 2 had the same composition as Sample 1 of Table I. The FIGURE 1 specimen was given a simulated box anneal for 20 hours at 875 F.

and the FIGURE 2 material was exposed to a simulated continuous anneal at 1075 F. for 20 seconds.

By conducting a series of tests, as are reported in Table I, it has been determined that optimum box an- TABLE I Sample 1 (0.03% O, 0.08% Mn) Sample 2 (0.04% C, 0.18% Mn) TAmlealing A l emperature nnea ing Percent Recrystal- Percent Rec ta F. Time Hardness,* lization Hardness,* zatiorl 1 R 30-1 R 30-T Rim Core Rim Core Control 77. 9 0 0 79. 0 0 I 0 800 74. 8 0 0 7 7. 5 0 0 74. 5 0 0 76. 5 0 0 *Eaeh hardness value is the average of five measurements.

nealing conditions include annealing temperatures of 875 to 925 F. for time periods (holding time at temperature) of 16 to 24 hours. The optimum conditions for continuous annealing have been found to be within a temperature range of from about l050 to 1100 F. for a time period of at least seconds, preferably to seconds. Table 11 below summarizes a series of tests to determine the recrystallization response to short time (20 seconds) annealing cycles. Steels of the same compositions as Samples 1 and Samples 2 of Table I were used in these tests. It is noted that the sample having the limited carbon and manganese content (sample 1) is preferentially surface recrystallized and recrystallization of the core does not occur until the rim is substantially fully recrystallized. In contrast, Sample 2 does not undergo preferential surface recrystallization but instead experiences random recrystallization occurring as frequently at the core as at the rim.

6 lized strip remains at satisfactorily high levels despite annealing to obtain improvement in ductility. Tensile strength of ordinary annealed black plate (not in Table III) is only 60,000 to 65,000 p.s.i., Whereas the tensile strength of the surface recrystallized strip is above 75,000 p.s.i. and may be as high as 88,000 p.s.i.

The steel strip produced according to the invention is particularly well suited for the manufacture of tin plate and a tin coating can be applied to the strip acoording to the usual practices.

I claim:

1. A method of making steel strip with a ductile substantially recrystallized surface and a fibrous substantially unrecrystallized core comprising cold reducing rimmed steel with a maximum carbon content of 0.05% and a maximum manganese content of 0.15% so as to produce a cold reduced, rimmed steel strip having a surface layer of metal purer than the core of said strip TABLE II Sample 1 (0.03% C, 0.08% Mn) Sample 2 (0.04% C, 0.18% Mn) Annealing Tempera Percent Recrystal- Percent Recrystaltu1e, F. Hardnes, lization Hardness. lization R 30-T R 30-1 Rim Core Rim Core 1 Samples held at temperature for 20 seconds.

One outstanding advantage of the surface recrystallized rimmed steel strip produced according to the invention is the increased ductility obtained with minimum sacrifice of tensile strength. The improved ductility is illustrated in Table HI which reports the number of reversed bends which samples of different strip types could withstand before showing first signs of fracture. The steel Sample A in the full hard condition was made by cold rolling without subsequent annealing. Samples B and C were subjected to simulated box annealing or continuous annealing according to the invention and Sample D was a double reduced tin plate which was given a conventional intermediate annealing between first and second cold reductions.

1 Longitudinal direction. 2 Transverse directlon.

A. Full hard (unanuealed condition).

B. Simulated box anneal 875 F., 20 hours.

C. Simulated continuous anneal 1,075 F., 20 seconds. D. Standard double-reduced 'tin plate.

It can also be seen from Table III above that the tensile strength of the preferentially surface recrystaland annealing said cold reduced strip at a temperature in the range of 800 to 1150 for a time sutficient to substantially recrystallize the surface layer of said strip but insuflicient to substantially recrystallize the cold reduced core of said strip.

2. A method according to claim 1 wherein said strip is batch annealed at a temperature in the range of 875 to 925 F. for 16 to 24 hours.

3. A method according to claim 1 wherein said strip is continuously annealed at a temperature in the range of 1050 to 1100 F. for 10 to 30 seconds.

4. In a method of making steel strip for tin plate, the improvement comprising cold reducing rimmed steel with a maximum carbon content of 0.05 and a maximum manganese content of 0.15% so as to produce a cold reduced, rimmed steel strip having a surface layer of metal purer than the core of said strip and annealing said cold reduced strip at a temperature in the range of 800 to 1150 F. for a time sutficient to substantially recrystallize the surface layer of said strip but insufficient to substantially recrystallize the cold reduced core of said strip.

5. A method of making improved tin plate comprising hot rolling a slab of rimmed steel with a maximum carbon content of 0.05% and a maximum manganese content of 0.15%, cold reducing said hot rolled rimmed steel so as to produce a cold reduced, rimmed steel strip having a surface layer of metal purer than the core of said strip, annealing said cold reduced strip at a temperature in the range of 800 to 1150 F. for a time sufficient to substantially recrystallize the surface layer of said strip but insufli-cient to substantially recrystallize the cold reduced core of said strip, cleaning the surface 7 8 of said strip to prepare it for tin coating and applying References Cited a coating of tin on said strip. 7 UNITED STATES PATENTS 6. A strip of rimmed steel having a maximum carbon content of 0.05% and a maximum manganese content of ,3 10/1962 M l r 1 816 0.15% with a ductile, substantially recrystallized surface 5 3,095,361 6/1963 Stone 20'429 X layer and a cold reduced, fibrous substantially unrecrys- 3,264,144 8/1966 Frazier et a1. 148l2 tallized core.

7. A strip of rimmed steel according to claim 6 where- DAVID L. RECK, Primary Examiner. in said surface layer is at least 50% recrystallized.

8. A strip of rimmed steel according to claim 6 where- HYLAND B Exammer' in said core is at least 70% unrecrystallized. H. F. SAlTO, Assistant Examiner. 

1. A METHOD OF MAKING STEEL STRIP WITH A DACTILE SUBSTANTIALLY RECRYSTALLIZED SURFACE AND A FIBROUS SUBSTANTIALLY UNRECRYSTALLIZED CORE COMPRISING COLD REDUCING RIMMED STEEL WITH A MAXIMUM CARBON CONTENT OF 0.05% AND A MAXIMUM MANGANESE CONTEND OF 0.15% SO AS TO PRODUCE A COLD REDUCED, RIMMED STEEL STRIP HAVING A SURFACE LAYER OF METAL PURER THAN THE CORE OF SAID STRIP AND ANNEALING SAID COLD REDUCED STRIP AT A TEMPERATURE IN THE RANGE OF 800* TO 1150*F. FOR A TIME SUFFICIENT TO SUBSTANTIALLY RECRYSTALLIZE THE SURFACE LAYER OF SAID STRIP BUT INSUFFICIENT TO SUBSTANTIALLY RECRYSTALLIZE THE COLD REDUCED CORE OF SAID STRIP. 